The present invention relates to a disconnect device for removing the driving connection between two rotating elements.
It is known to provide disconnect devices between a prime mover and a load driven from a prime mover. Such disconnect devices are often found in aircraft generator systems where an aircraft generator is driven from a spool of a gas turbine engine. It is highly desirable that, in the event of a generator malfunction or upon detection of conditions which are indicative of an impending generator malfunction, that the generator load can be disconnected from the prime mover thereby ensuring that the operation of the prime mover is unpeturbed by the fault in the generator.
U.S. Pat. No. 5,103,949 discloses a disconnection device in which a drive shaft carries a threaded portion and a plunger is arranged to move in from the side of the drive shaft to engage the threaded portion, thereby causing the screwing action between the plunger and the threaded portion to displace the shaft axially such that it becomes disconnected from a prime mover.
U.S. Pat. No. 4,042,088 similarly has a drive shaft which engages a prime mover via a castellated connection region. The drive shaft also carries a helically threaded portion and a disconnect plunger is arranged to move in from the side of the drive shaft to engage the threaded portion to cause the shaft to move axially so as to disconnect the shaft from the prime mover.
U.S. Pat. No. 4,989,707 discloses a similar arrangement in which a element moves in from the side of a drive interface between input and output shafts in order to engage a threaded portion of a ring in order to cause disconnection between the shafts.
Each of these prior art arrangements suffers from the problem that the shafts will, in general, be rotating rapidly and that the teeth on the control element which moves radially inward to engage the screw thread must engage the thread rapidly before they become damaged or sheared off.
U.S. Pat. No. 4,086,991 discloses a disconnect coupling in which helical splines are used to connect a coupling shaft to a driven member so as to transmit torque to the driven member. The helical splines are carried on an axially movable shaft and are arranged such that the transmission of torque to the driven member acts to urge the coupling shaft and driven member to move to a disengaged position. In normal use, this movement to a disengaged position is inhibited by the provision of a fusible element, such as a eutectic pellet. Such pellets are generally of a soft material and the crushing of the pellet under a compressive load is a well recognised problem, see for example, U.S. Pat. No. 4,271,947 wherein the pellet is manufactured with wire strands therein in order to give it additional mechanical strength. Because of the use of helical splines in U.S. Pat. No. 4,086,991 the crushing force acting on the eutectic pellet varies as a function of the torque transmitted through the disconnect coupling. In particular, the pellet must be able to withstand the crushing load at full torque transfer without suffering deformation. This increase in material in the pellet means that the pellet has an increased mass and thermal capacity, and as such the rate at which the pellet warms is reduced, thereby leading to a potential slowing of the decoupling mechanism. Another problem with the system described in U.S. Pat. No. 4,086,991 is that decoupling between the shaft and the load may not occur when the shaft is lightly loaded. This is significant since the load may be a generator and it is conceivable that the generator itself may not fail, but that the cooling system for the generator might fail, thereby resulting in the need to disconnect the generator in order to prevent damage to it even when the generator is lightly loaded.
U.S. Pat. No. 4,271,947 discloses an arrangement in which two axially aligned shafts engage each other via coaxial gears having teeth extending in the axial direction. A compression spring extends between the gears and urges an axially displaceable one of the gears to move away from the axially fixed gear. A fusible element having strengthening filaments therein acts to resist both the force of the compression spring and the axial forces resulting from torque transfer via the inclined surfaces of the gear teeth.
U.S. Pat. No. 4,685,550 discloses a disconnect device in which opposing rings of teeth can move apart axially to disconnect a motor from an output shaft. This connection motion is inhibited by a ball detent mechanism. A solenoid can be operated in order to open an escape route for the ball such that it falls radially inwards into a central shaft. This then allows the rows of teeth (which form part of a tapered jaw clutch) to disconnect.
According to the present invention, there is provided a selectively disconnectable drive coupling comprising a first element, a second element, a plurality of coupling elements and a release mechanism, wherein the first and second elements are coaxially aligned with a portion of the first element extending within a channel formed in the second element, and the coupling elements are moveable between a drive position where they extend between seats in the first element and corresponding passages in the second element such that the first and the second elements are drivingly connected, and a disconnected position in which the plurality of coupling elements have moved out of the passages in the second element so as to drivingly disconnect the first and second elements, and wherein the release mechanism retains the coupling elements at the drive position until such time as the release mechanism is activated.
It is thus possible to provide a quick release disconnect mechanism in which the coupling elements move towards the disconnected position under a combination of the forces acting between the coupling elements and the first element and also centrifugal force due to the rotary motion of the first and second elements.
Preferably the first element is an input shaft and the second element is an output shaft or other device driven by the input shaft.
Advantageously the coupling elements have curved or inclined surfaces that engage with co-operating surfaces in the seats on the first element such that the transmission of torque between the first element and the second element acts to urge the coupling elements in the disconnected direction.
Preferably the release mechanism comprises a keeper element which blocks the motion of the coupling elements from the drive position to the disconnect position until such time as a release instruction is received.
Preferably the blocking action of the keeper element is removed in a response to an electrical signal.
Advantageously the keeper element is axially slideable along an exterior surface of the second element. The keeper element can thus be removed from the position at which it blocks the motion of the coupling elements from the drive position to the disconnect position.
Preferably the keeper element includes at least one capture region, for example at least one cavity or channel therein, in order to capture the coupling elements after they have moved to the disconnect position. Advantageously, after the keeper has entrained the coupling elements within its at least one cavity or channel, the keeper moves to a further position whereby the coupling elements are prevented from re-entering the passages in the second element, thereby ensuring that drive cannot be inadvertently restored.
Preferably the keeper element is spring biased from the drive position to a decoupled position.
Advantageously the keeper is held against the urging of the spring bias by a release element. Advantageously the release element is responsive to the presence of an external signal to release the keeper element such that it becomes free to move from the blocking position.
Advantageously the release element is responsive to the presence of a magnetic field.
Preferably the release element is a fusible element which becomes melted as a result of eddy current heating occurring as a result of the presence of the magnetic field. In a preferred embodiment, the release element is a ring of metal having a relatively low melting point, such as solder or a eutectic mix, such that eddy current heating occurring as a result of the rotary motion of the release element with respect to the magnetic field causes the release element to melt thereby allowing the keeper element to move under the urging of its spring.
Advantageously the release element is contained within a capture element, for example in the form of a cylindrical shell or a cup. This constrains the melted material of the release element from being thrown outwards under centrifugal force and also provides an additional source of heating. Advantageously the keeper element extends within the cylindrical or cup shaped element, even when the keeper element is at the drive position. Advantageously the opposing end of the cup or cylinder opens into a capture region where the melted material of the release element becomes retained. Thus, as the release element melts, the melted material moves into the capture area, either as a result of the flow of the material under centrifugal force or as a result of being squeezed because of the motion of the keeper element.
Advantageously one of the first and second elements includes a weakened section such that this section sheers at a predetermined torque in order to form a further point of disconnect in the event that a sudden seizure results in unexpectedly high drive forces occurring.